1. Field of the Invention
This invention relates to a projection device using a plurality of bar lenses of plastic, the optical performance of which is not varied by humidity changes and the setting of which can be effected easily and accurately.
2. Description of the Prior Art
Plastic materials have a great water absorbing property or a great dewatering property and lenses formed of plastic materials have suffered from a problem such that the optical performance thereof, especially the refractive index distribution in the lenses, is varied by variations in environmental humidity. More particularly, in the refractive index distribution in a plane perpendicular to the optical axis of the plastic lenses, if the environmental humidity increases, the refractive index in the central portion tends to decrease as compared with the refractive index in the marginal portion, although the difference is not remarkable, and if the environmental humidity decreases, the refractive index in the marginal portion tends to decrease as compared with the refractive index in the central portion and therefore, when light rays pass through the lens, the light rays which originally traveled rectilinearly are bent due to the difference in the refractive index within the lens, causing non-projection of the image onto a predetermined image forming position.
The plastic employed in the invention is a lens having a greater length in the direction of the optical axis as compared with its effective diameter, as shown in our prior U.S. Application Ser. No. 889,404, now abandoned (hereinafter referred to as the bar lens). In such a lens, the distance between conjugate images in an ommateal optical system wherein a part area of the object surface is covered by a pair of bar lenses can be reduced to thereby provide a compact copying machine. Since an ommateal optical system uses a number of bar lenses, it is therefore desired that these bar lenses can be accurately and easily set in a holding member.